1. Field of the Invention
The invention relates to a hybrid electric vehicle powertrain with a multiple-ratio transmission for delivering driving power to vehicle traction wheels.
2. Background Art
Hybrid electric vehicle powertrains for automotive vehicles typically include an internal combustion engine and an electric motor-generator, which define mechanical and electrical driving power sources. One class of hybrid electric vehicle powertrains includes an engine and a motor-generator with gearing in a split power arrangement in which power delivery from each power source to vehicle traction wheels is coordinated to meet a driver demand for power. Such a powertrain arrangement is disclosed in U.S. Pat. No. 6,991,053, which is assigned to the assignee of the present invention. Another class of hybrid electric vehicle powertrains is a so-called dual-drive hybrid electric vehicle powertrain in which an internal combustion engine and a multiple-ratio automatic transmission are situated on a common axis and a crankshaft integrated starter-generator is disposed between the engine and a torque input shaft for the transmission. An example of a powertrain of this type is disclosed in U.S. patent application Ser. No. 11/467,588, filed Aug. 28, 2006, entitled “Strategy for Improving Shift Quality in a Hybrid Electric Vehicle Powertrain,” which is assigned to the assignee of the present invention. An example of a dual-drive hybrid electric vehicle powertrain with a disconnect clutch between the engine and an electric motor is disclosed in U.S. Pat. No. 6,116,363.
Still another example of a hybrid electric vehicle powertrain having a multiple-ratio automatic transmission, an electric traction motor and an internal combustion engine defining dual power flow paths to vehicle traction wheels is disclosed in U.S. patent publication 2005/0054480, filed Sep. 10, 2004, which also is assigned to the assignee of the present invention.
A hybrid electric vehicle powertrain having a standard manually-controlled transmission with a conventional neutral clutch in a powertrain that includes an engine and an electric motor is disclosed in U.S. Pat. No. 6,319,168.
The dual-drive powertrains disclosed in the latter two of these prior references have controlled ratio upshifts as the powertrain accelerates the vehicle. Electric motor torque and engine torque under the control of a vehicle system controller develop traction wheel torque in an upshift interval.
The patent publication, identified above, discloses a hybrid electric powertrain with a multiple ratio transmission having a hydrokinetic torque converter and multiple-ratio gearing in the torque flow path from the engine to the traction wheels. That powertrain includes a slipping wet clutch to connect a crankshaft of the internal combustion engine to the torque input shaft of the transmission. The engagement characteristics of the wet clutch can be used to attenuate inertia torque fluctuations during a transmission upshift or a coasting downshift.
In the case of power-on downshifts of known hybrid electric vehicle powertrains, the vehicle driver typically depresses an accelerator pedal to request torque to accelerate the vehicle. A transmission controller will respond to a driver request for torque by downshifting the transmission, which causes the engine torque to increase because of the increased throttle setting that accompanies depression of the accelerator pedal. The first increment of the engine torque increase is used to overcome the engine inertia torque and to bring the engine speed up to the higher level required with an increased transmission gear ratio.
During a downshift, the transmission will remain in the upshifted torque ratio until the end of the speed ratio change. During the progression of the speed ratio change, there is no increase in wheel torque since the increase in engine torque is used to overcome inertia forces. This condition causes a delay in the desired acceleration increase. The length of this delay depends on the amount of torque that the engine is producing for a given amount of effective inertia of the rotating elements of the engine and the powertrain. Typically, this delay time may be close to one second.